Method of preparing membranes from blends of polymers

ABSTRACT

Improved anisotropic fluid separation membranes are prepared from blends of polymers with surface energy differences. The membranes are formulated by processes wherein low surface energy polymer with desirable fluid separation and permeation characteristics is preferentially concentrated in the surface discriminating layer of the membrane.

This application is a Continuation of prior U.S. application Ser. No.08/320,725 Filing Date Oct. 11, 1994, now U.S. Pat. No. 5,733,657.

FIELD OF THE INVENTION

This invention describes improved anisotropic fluid separation membranesand a method of producing these membranes from solutions containingblends of polymers. The anisotropic membranes of this invention areparticularly useful for gas separations.

BACKGROUND OF THE INVENTION

Processes for making synthetic polymeric membranes, including hollowfiber gas separation membranes, are well documented in the art.Separation/permeation characteristics of membranes are optimized withrespect to intended end use. Consequently, consideration must be givento materials and manufacturing methods to be employed in membranemanufacturing.

A superior membrane must have a good balance of chemical, mechanical,and separation characteristics in order to function properly. It isoften not possible, however, to obtain high performance in all aspectsof membrane properties from a single material. Thus, a method todecouple the membrane separation/permeation characteristics from thebulk mechanical properties is frequently needed.

One method extensively employed in the art to decouple mechanicalproperties from membrane separation characteristics is via the compositemembrane approach, wherein a thin separation layer is deposited by asolution coating method on a preformed substrate. Preparation of suchmembranes is described in U.S. Pat. Nos. 4,243,701; 4,826,599 and4,840,819.

Another method that accomplishes this goal is coextrusion. Twodistinctly different polymer solutions are coextruded simultaneously toform a bilayer hollow fiber. The technique permits the active sheathlayer to be formed from a polymer with superior separation/permeationcharacteristics while the core layer that makes up the majority ofmembrane mass is formed from a common polymer with good mechanical andthermal characteristics. Examples of this method are taught by Ekiner etal. in U.S. Pat. No. 5,085,676 and by Kusuki et al. in Japanese PatentApplication No. Sho 62-253785.

Yet another method used to optimize membrane properties employs castingsolutions containing blends of two or more polymers. There are numerousexamples in the art of fluid separation membranes advantageouslyprepared from blends of polymers. Kraus et al. in U.S. Pat. No.5,076,935 teach the use of polyethersulfone/phenoxy resin blends to makeporous isotropic membranes. Nunes et al. describe preparation ofasymmetric membranes useful for ultrafiltration from blends ofpolyvinylidene fluoride and poly(methyl methacrylate) in the Journal ofMembrane Science, 73(1992), 25-35. The practice of blending polymersalso has been used effectively in the preparation of gas separationmembranes. Yamada et al. in U.S. Pat. No. 4,832,713 disclose fabricationof gas separation membranes from blends of polyetherimide mixed withmaterials such as polycarbonates or polysulfones. Kohn et al. in U.S.Pat. No. 5,055,116 utilize miscible blends of polyimides of specificchemical compositions to prepare gas separation membranes. Burgoyne, Jr.et al. in U.S. Pat. No. 5,061,298 also describe preparation of membranesfrom specific polyimide blends.

Each of the aforementioned methods of membrane preparation, however, hassome limitations. Coextrusion is a relatively complicated processbecause of the need for costly, specialized hardware such asdual-annulus spinnerettes and the need to formulate two spinning dopesin order to prepare a hollow fiber membrane. Preparation of compositemembranes by solution coating methods is a two-step process whereinselection of the coating separation material is frequently limited bythe solvent resistance characteristics of the substrate. Polymerblending techniques have been optimized around polymers with specificchemical structures. Polymers frequently have to be thermodynamicallycompatible, i.e. miscible, in order to form superior membranes. Thusthere still exists a need for a simple, efficient method ofmanufacturing membranes with improved combination of mechanical andseparation properties.

SUMMARY OF THE INVENTION

In one embodiment, the invention provides an anisotropic fluidseparation membrane comprised of a blend of two or more polymers whereinat least one low surface energy polymer that comprises less than 20percent by weight of the overall membrane material is preferentiallyconcentrated in the surface discriminating layer of said fluidseparation membrane.

In another embodiment, the invention provides an anisotropic gasseparation hollow fiber membrane having a surface discriminating layerless than 1000Å in thickness, said membrane being prepared by a processcomprising the steps of:

a) forming a mixture of solvent and two or more polymers, wherein atleast one of said polymers is a low surface energy polymer having asurface energy at least 10% less than at least one other polymer of saidmixture and wherein said low surface energy polymer comprises less than20% by weight of the overall polymer composition;

b) extruding the polymer-solvent mixture in the form of a hollow fiberthrough a gaseous atmosphere and then into a liquid medium wherein saidfiber solidifies;

c) washing the solidified hollow fiber; and

d) recovering the solidified highly anisotropic hollow fiber.

In another embodiment, the invention provides an anisotropic fluidseparation membrane comprising a blend of two or more polymers whereinthe concentration of at least one polymeric component in a surfacediscriminating layer of said anisotropic fluid separation membrane ismade higher than the bulk concentration of said component in the blendmembrane by annealing said fluid separation membrane at an elevatedtemperature of from 20° C. to 1° C. less than the glass transitiontemperature of the bulk polymer component, said preferentiallyconcentrated component comprising less than 20 percent by weight of theoverall membrane composition.

DETAILED DESCRIPTION OF THE INVENTION

It has been found surprisingly that improved anisotropic fluidseparation membranes can be formed in a single-step process from acasting solution of polymer blends wherein the polymeric ingredients ofthis casting solution are preselected as to have significant surfaceenergy differences. By selecting polymers with differences in surfaceenergies, it is possible to form, in a single step, anisotropic fluidseparation membranes wherein the membrane surface layer is enriched withrespect to the low surface energy polymeric component. A system of twopolymers can be chosen such that one of the materials will form adisproportionately high concentration of the membrane surface layercomposition even though comprising a lesser fraction of casting solutioncomposition. By further selecting a low surface energy polymericcomponent with separation/permeation or chemical structure beneficialfor a particular fluid separation application, especially at the surfacelayer, it is possible to obtain improved separation membranes.

Membranes of this invention are cast from an appropriate polymer blendsolution by dry/wet phase inversion processes. The process can be usedto produce membranes in any desired configuration such as flat sheet,tubular or spiral wound configuration, but it is preferred to formmembranes of this invention in the form of hollow fibers.

The blend membranes of the prior art are typically made of polymers thatare miscible in a common membrane forming solvent system and frequentlyare further mutually miscible. It is not necessary for the purposes ofthis invention for membrane forming polymers to be truly miscible. Theuse of a spin dope that is homogenous in nature at the time of spinningis sufficient to form membranes according to this invention.Appropriately formulated spin dopes of this invention will containingredients that will maintain stability of the spinning dope for anextended period of time as a result of high viscosity and chemicalinteractions.

This invention can thus be used to produce membranes with improvedproperties from casting solutions that contain polymers with significantdifferences in surface energy characteristics wherein the castingsolution will contain a low surface energy polymer in a small amountrelative to the amount of the high surface energy polymer. Since asolution will tend to minimize its surface energy, the migration of thelower surface energy polymer to the surface of the solution isthermodynamically favorable. The low surface energy material isconsequently expected to enrich the surface of the membrane formingsolution prior to coagulation. The phase change that takes place duringcoagulation serves to fix the low surface energy polymer at the membranesurface at a level disproportionately higher than its bulk concentrationin the membrane.

Surface energy of polymers is usually ascertained by measuring polymersurface tension. Methods of determining the surface tension, (e.g.surface energy) of solids are well known in the art. Examples of suchmethods include measuring contact angle between the solid and differentliquids, measurement of Zisman's critical surface tension andextrapolation of surface tension data of polymer melts to roomtemperature. Descriptions of these methods can be found in D. W. VanKrevelen, Properties of Polymers, Elsevier, 1976; R. J. Good, Surfaceand Colloid Science, 11 (1979), pp.1-29; D. K. Owens and R. C. Wendt,Journal of Applied Polymer Science, 13 (1969), pp.1741-1747; and D. H.Kaelble, Polymer Engineering and Science, 17 (1977), pp. 474-477. Themeasured surface energy, i.e. surface tension, is defined herein asmeasured by any of the methods described above. The surface energy ofindividual polymeric components that form the membrane thus can bemeasured to determine their respective values. By the term low surfaceenergy polymer, it is meant that the measured surface energy of thispolymer is at least 10 percent lower than the measured surface energy ofthe remaining polymers of the blend, the latter polymers comprising thebulk of the membrane composition. A difference in the surface energiesof the membrane forming polymers of about 10% can cause a significantconcentration of the low surface energy polymer in the surface of theanisotropic membrane. A difference of about 20, is preferable and adifference of 40% or more is most preferable. This low surface energypolymeric component can decrease the surface energy of the blend by asmuch as 2 dynes/cm, preferably by 5 dynes/cm or more. The significantdecrease in the surface energy of the membranes is the primary reasonfor the preferential concentration of this minor polymeric component inthe anisotropic surface layer. Preferential concentration of the lowsurface energy component in the surface discriminating layer can bedirectly confirmed by surface analysis methods such as ATR-FTIR(Attenuated Reflectance Fourier Transform Infrared Spectroscopy), ESCA,SIMS, and ISS among others. It is also possible to confirm that the lowsurface energy polymer is concentrated in the membrane surface layerindirectly by surface energy measurements. The surface energy of themembrane should be lowered substantially by incorporation of the lowsurface energy polymer into the blend. The measured values should beclose to the surface energy value of the low surface energy componentand should not exceed this value by more than 20 percent of thedifference between the surface energy values of the low surface energycomponent and the sum of the other polymeric components of the blend.The concentration of the polymer (in weight percent) that lowers thesurface energy of the blend and preferentially concentrates in thesurface discriminating layer to the bulk high surface energy polymer, orpolymers, is typically less than 20 percent, preferably less than 10percent, and most preferably from 5 to 0.5 percent. In some embodimentsthe concentration may be as low as 0.25 percent or less. Despite the lowbulk concentration of the low surface energy polymer in the blend, itsconcentration in the surface discriminating layer is much higher thanthe bulk and can be above 20 percent or preferably above 50 percent ofthe composition of the surface discriminating layer (e.g. the outer1000Å).

It is understood that the polymer blend composition can be comprised ofmore than two polymeric ingredients, as long as at least one polymericcomponent that constitutes a minor fraction of the overall polymericcomposition exhibits the characteristic of significantly decreasing theoverall surface energy of the blend. There may be cases when more thantwo polymeric components are incorporated into the polymeric blend. Thismay include more than one low surface energy polymeric component. Thecomponents may form mutually miscible or immiscible blends. The blendscan further include two or more high surface energy components that formthe bulk of the membrane structure. These components can also formmutually miscible or immiscible blends.

The fact that minor amounts of the low surface energy polymer in thecasting solution can be used according to the method of this inventionto modify an anisotropic membrane surface layer offers a distinctadvantage over processes described in the prior art. It is now possibleto decouple the characteristics of the active membrane surface from theproperties of the membrane substructure. This can be accomplished byformulating the casting dope with two polymers having sufficient surfaceenergy differences. The low surface energy polymer can be used tocontrol the permeation and separation characteristics of the membranebecause the material is preferentially concentrated in thediscriminating layer, while the high surface energy polymer material canbe used to impart desirable characteristics such as mechanical strengthto the substructure of the membrane. It will be apparent to one skilledin the art that more expensive, designer polymers can now be employed tocontrol the surface characteristics of membranes in an economicalmanner.

In order to produce a hollow fiber membrane through a phase inversionprocess, it is necessary to properly formulate a spinning solution ordope from which the fiber is formed. The spin dope is typically a highviscosity, homogeneous mixture of membrane forming polymer, solvents,and various additives. Each of the ingredients of the spin dope caninfluence the spinnability of the dope and subsequently the finalproperties of the resulting membranes. Methods to formulate solutions ordopes for spinning hollow fibers are well known to those skilled in theart. The principles that apply toward design of a membrane formingsolution utilizing a single polymer apply to formulation of polymerblend solutions as well.

In a preferred embodiment of this invention the membranes are spun intohollow fiber configuration by the process described by Bikson et al. inU.S. Pat. No. 5,181,940. This patent teaches a method of producinghighly anisotropic hollow fibers useful as permselective gas separationmembranes and as substrates for preparation of composite membranes byextruding a spinning solution through a tube-in-orifice spinnerette intoa gas filled chamber maintained at reduced pressure followed bycoagulation and solidification step. Spinning solutions formulated fromblends of polymers with different surface energies can be advantageouslyspun into hollow fibers utilizing this spinning method. It has beenfound that only minor amounts of polymer that lower the surface energyof the blend in the spinning solution formulation are required toproduce hollow fiber membranes with substantially improved fluidseparation characteristics.

The distinguishing feature of blend membranes of the present inventionis the fact that they are anisotropic and contain an integraldiscriminating layer frequently referred in the art as the skin. Thislayer can be less than 1000Å thick, preferably less than 500Å thick,most preferably less than 250Å thick. The discriminating layer isdistinguishable from the main membrane body by somewhat decreasedporosity, i.e. increased density and/or somewhat decreased porediameter. The porosity of the discriminating layer (porosity is definedas the ratio of the area occupied by pores to the total area of thediscriminating layer) will vary from about high 10⁻² range to below 10⁻⁵-10⁻⁶. Low porosity is most desirable for integral asymmetric membranes,in particular integral asymmetric gas separation membranes, while highlevels of surface porosity are particularly useful for preparation ofcomposite membranes. The discriminating layer is typically located atthe exterior membrane surface. The hollow fiber membranes may containthe discriminating layer at the exterior or the interior wall.

The additional distinguishing feature of the blend membranes of thepresent invention is the fact that the composition of the membrane issubstantially different throughout its structure, i.e. the surfacecomposition differs from the bulk composition.

The membranes of this invention are highly anisotropic and can beprepared with very thin discriminating layers preferably less than 250Åthick. Discriminating layer thicknesses of these magnitudes can beadvantageously achieved by the aforementioned vacuum spinning technique.The hollow fiber wall morphology and the thickness of membranediscriminating layer can be further modified through the use ofcoagulants that may include such solvents as alcohols andsolvent/nonsolvent mixtures. However, the most often used coagulant iswater or mixtures of water with solvents, surfactants and salts.

The anisotropic membrane prepared by the phase inversion method can befurther treated by solvent exchange techniques well known in the art toimpart improved characteristics to the anisotropic surface layer. Thesurface discriminating layer of the blend membranes of this inventioncan be further modified by high temperature annealing techniques. Themethod described in U.S. Pat. No. 4,881,954 is particularly useful inmodifying the surface characteristics of the blend membranes of thisinvention. The method provides for high temperature annealing of theanisotropic membrane of this invention at temperatures slightly belowthe glass transition temperature of the polymeric component thatcomprises the bulk of membrane composition, frequently from 20° C. toabout 1° C. less than the glass transition temperature of the bulkpolymeric component. The annealing temperature can be lower orpreferably higher than the glass transition temperature of the minorpolymeric component of the blend that decreases the overall surfaceenergy of the blend. Enrichment of the anisotropic membrane layer withthe minor component thus may take place during the high temperatureannealing process. In certain embodiments of this invention wherein thepolymeric blend components are miscible and the surface energydifferences between the components are small, the high temperatureannealing step may be required to induce significant enrichment of thesurface discriminating layer by the minor component of the blend thatdecreases the overall surface energy of the membrane.

The anisotropic membranes of this invention are uniquely suited forpreparation of composite and multicomponent gas separation membranes.These membranes can be advantageously prepared by solution coatingmethods. Examples of such methods are shown in U.S. Pat. Nos. 5,076,916;4,840,819; 4,826,599; 4,756,932 and 4,467,001. The coating is depositedonto the discriminating layer of the membrane and in some embodimentscan partially or completely occlude the pores. The coating material, thecoating morphology and coating thicknesses can be selected by thoseskilled in the art to meet the needs of specific gas separationapplications. Dense ultra-thin coatings as thin as 500Å or less can besuccessfully formed on the surfaces of the blend membranes of thisinvention by solution deposition methods. A broad range of solvents canbe utilized in the preparation of coated membranes. The coating solventselection is governed by coating film forming requirements and substratesolvent resistance characteristics.

In one embodiment of this invention high surface porosity hollow fibers(i.e. hollow fibers with high discriminating layer porosity) areadvantageously produced from blends of polymers with significant surfaceenergy differences. Such membranes can be utilized directly in fluidseparation applications such as ultrafiltration or as substrates formanufacturing of composite fluid separation membranes. In one embodimenthollow fibers are coated with high gas permeability materials. Thesecoated membranes may be useful for gas and vapor separation applicationssuch as oxygen enrichment or organic vapor removal from air. Coatingmaterials that can be advantageously employed to prepare compositemembranes of this type include siloxanes such as poly(dimethylsiloxane),polybutadiene and ethylene-propylene-diene monomer (EPDM) rubbers andthe like. In another embodiment, it may be desirable to coat the highsurface porosity hollow fibers with a high gas separation factor glassypolymer, which to a large extent determines the gas separationcharacteristics of the composite membrane. Examples of such materialsinclude sulfonated polyarylethers, sulfonated poly(phenylene oxides),polyesters, polyestercarbonates, and cellulosic derivative polymers suchas cellulose acetate and blends of cellulose acetate with poly(methylmethacrylate) to name a few. Detailed description of chemical structureand preparation methods for some of these materials can be found in U.S.Pat. Nos. 5,071,498; 5,055,114; 4,994,095; 4,971,695; 4,919,865; and4,874,401. These composite membranes are most suitable for airseparation applications, acid gas separations, or hydrogen/methaneseparations. Composite membranes such as these can occasionally haveminor defects that can be further repaired by post-treatment methodswith solvents, dilute solutions of polymers and reactive additives.Post-treatment procedures of this type are taught by Bikson et al. inU.S. Pat. Nos. 4,767,422 and 5,131,927.

In another embodiment of this invention low discriminating layerporosity hollow fiber membranes are produced from polymer blends of thisinvention. Such membranes can be utilized directly for fluid separationsor further coated prior to use. In some embodiments such as gasseparation applications, the dry-wet spun hollow fiber membranes aredried prior to use by air drying or other prior art processes. Forexample, membranes spun into water baths can be dehydrated by methodsshown in U.S. Pat. Nos. 4,080,743 and 4,120,098. In another embodimentit may be desirable to overcoat these membranes with a high gaspermeability material such as silicone rubber to repair residual defectsin the membrane separation layer prior to use. High gas permeability,low separation factor elastomeric coatings are frequently used to repairminor defects that occur in highly asymmetric low surface porositymembranes. Preparation of such multicomponent gas separation membranesis described in U.S. Pat. No. 4,230,463. In other cases, it may beadvantageous to coat these low surface porosity hollow fibers with highgas separation factor materials that contribute to the overall gasseparation characteristics of the composite membrane. These high gasseparation factor materials are frequently glassy polymers.Representative examples of such polymers include polyesters,polyestercarbonates, sulfonated polysulfones and sulfonatedpoly(phenylene oxides), cellulosic derivative polymers, such ascellulose acetate or blends of cellulose acetate with poly(methylmethacrylate) to name a few. Coating of these glassy polymers onto lowsurface porosity hollow fibers often yields an essentially defect-freecomposite gas separation membrane with an attractive combination ofpermeation and separation characteristics.

Polymers that lower the surface energy of the anisotropic membrane layerand are particularly useful as additives in blend membranes of thisinvention include polymers containing siloxane groups, in particulardimethylsiloxane groups, and perfluorohydrocarbon groups, in particularCF₃ and CF₂ groups. In particular, useful polymers are block and graftcopolymers that contain poly(dimethylsiloxane) segments. Low surfaceenergy polymers include rigid backbone and flexible backbone polymers,including some copolymers comprised of alternating rigid and flexiblesegments. Examples of low surface energy polymers useful for preparationof gas separation membranes are polyimides, including polyetherimides,polyesters, polycarbonates and polyestercarbonates that contain lowsurface energy groups/segments.

The high surface energy polymers than can be advantageously utilized inthe blends of the present invention include polysulfones, such aspolyarylether sulfones and polyether sulfones; polyesters; polyimides,including polyetherimides; polycarbonates; cellulosic derivativepolymers, such as cellulose acetate, polyamides, polyimideamides andpolybenzimidazoles.

In a specific embodiment of this invention, gas separation membraneshave been fabricated from two commercially available polymers sold underthe trade names of Ultem® 1000 and Siltem® D9000. Both materials aremanufactured by General Electric Plastics Co. The former material is apolyetherimide and the latter material is described by the manufactureras a siloxane polyetherimide copolymer. The siloxane polyetherimidecopolymer has substantially higher gas permeability coefficients thanthe polyetherimide polymer. For example, the oxygen permeabilitycoefficient of Siltem is 15.4 Barrer at 30° C., which is approximately40 times higher than that of Ultem; however, this high gas permeabilityis combined with somewhat lower gas separation characteristics.Membranes prepared from this material would thus be advantageous inapplications requiring high gas permeation rates. The tensile strengthof Siltem polymer, however, is only about 4100 psi. These properties maynot be adequate for preparation of asymmetric gas separation membranesby conventional processes. However, anisotropic membranes can beprepared from blends of Ultem and Siltem according to methods of thisinvention wherein these membranes exhibit advantageous gas permeationproperties combined with required mechanical strength.

This invention permits the use of such materials because the integrityof the resulting membrane comes from a high strength material such aspolyetherimide polymer. This polymer can constitute the vast majority,for example up to 99.5%, of the membrane mass. A very small amount, aslittle as 0.5% of the overall membrane mass, of a secondary polymer likeSiltem, is needed to impart beneficial properties to the anisotropicmembrane. Only small amounts of this low surface energy polymer arerequired because this material will be preferentially concentrated inthe anisotropic membrane layer during the casting process for thereasons expressed above. The Siltem polymer has a lower surface energy(16.7 dynes/cm) than the Ultem polymer (41.1 dynes/cm) that comprisesthe bulk of the membrane structure.

To demonstrate the reduction in the surface energy of polymer blendsthat occurs on incorporation of Siltem polymer, the surface energy ofUltem/Siltem blends, as well as its respective polymer components, wasdetermined by measuring dynamic contact angles. The results aresummarized in Table 1. It is apparent from the data in Table 1 thatincorporation of even small amounts of Siltem reduces the total surfaceenergy γ_(t) of the blends to the surface energy levels that arecomparable to that of pure Siltem polymer.

                  TABLE 1    ______________________________________                       Surface Energy γ.sub.t    Specimen           dynes/cm    ______________________________________    Ultem              41.1    Siltem             16.7    Ultem/Siltem blend, 9/1 ratio                       17.8    Ultem/Siltem blend, 166/1 ratio                       18.8    ______________________________________

To demonstrate the preferential concentration of Siltem polymer at thesurfaces of membranes cast from Siltem/Ultem blends, a film was castfrom the solution composed of 9 parts of Ultem polymer, 1 part of Siltempolymer and 90 parts of N-methyl pyrrolidone. The film was driedextensively and the atomic surface composition determined by ESCAanalysis. The results are summarized in Table 2 together with the atomiccompositions of Ultem and Siltem polymers. The results indicate that thesurface of the film cast from the blend of Siltem and Ultem polymers,wherein Siltem polymer comprises only a minor fraction of the overallblend composition, is composed essentially of Siltem polymer only.

                  TABLE 2    ______________________________________    Atomic composition of polyetherimide, siloxane    polyetherimide copolymer, and the surface composition    of their blend    Specimen    % Si    % C        % N  % O    ______________________________________    Ultem/Siltem                12.5    65.5       2.2  19.8    blend 9/1    ratio    Ultem resin trace   82.5       4.3  13                amount    Siltem resin                13      64.8       2.2  19.2    ______________________________________

This invention makes it possible to improve membrane performancecharacteristics by incorporating an appropriate low surface energypolymer having separation properties desirable of a surface polymer intothe casting solution. Since this polymer needs to be present in onlyminor amounts, a cost effective and simple method of controllingproperties of an anisotropic membrane without compromise of itsstructural integrity is now available. Moreover, concentration of thepreferred surface polymer at the surface occurs without the necessity orcost of coextrusion techniques.

The following examples serve to illustrate further the utility of thisinvention in the preparation of gas separation membranes fromUltem/Siltem blends but are not intended to be limiting.

EXAMPLE 1

A spinning dope consisting of 38.26 parts of Ultem 1000 polyetherimideresin, 0.24 parts of Siltem D9000 siloxane polyetherimide copolymer,15.0 parts of TRITON® X100, which is a non-ionic surfactant having thecomposition octyl-phenoxypolyethoxyethanol, and 46.5 parts of N-methylpyrrolidone was spun into hollow fibers. The spin dope was pumpedthrough a tube-in-orifice spinnerette having an-orifice diameter of0.1016 cm and an injection tube outside diameter of 0.0508 cm at a rateof 3.0 cc/min and at a temperature of 71° C. Simultaneously, a corefluid of gamma-butyrolactone was delivered to the core of the injectiontube at a rate of 1.2 cc/min to produce a hollow filament stream. Thespinnerette was completely enclosed in a vacuum chamber in which thevacuum level was 14 cmHg.

The hollow filament stream travelled through the vacuum chamber for adistance of 2.5 cm, whereupon it entered the top of a coagulation columnand was then drawn at a speed of 31.7 meters/min through a quench baththat consisted of 0.05% solution of TRITON® X100 in water maintained at45° C.

The resulting asymmetric hollow fiber had an outside diameter of about0.038 cm and an inside diameter of about 0.02 cm. The fibers were washedto remove residual solvent components, dried, and coated with a 6%solution of poly(dimethylsiloxane) in cyclohexane.

The coated fibers were heated to remove the cyclohexane solvent and thenfabricated into modules containing 8 hollow fibers about 40.5 cm long.The membranes were tested for air separation characteristics at apressure of 7.03 Kg/cm² and about 23° C.

EXAMPLES 2

Membranes were prepared and tested in a manner identical to thatdescribed in Example 1 except that the membrane polymer components inthe spinning dope consisted of 38.02 parts of Ultem 1000 and 0.48 partsof Siltem D9000.

EXAMPLE 3

Membranes were prepared and tested in a manner identical to thatdescribed in Example 1 except that the membrane polymer components inthe spin dope consisted of 37.54 parts of Ultem 1000 and 0.96 parts ofSiltem D9000.

EXAMPLE 4

Membranes were prepared and tested in a manner identical to thatdescribed in Example 1 except that the membrane polymer components inthe spin dope consisted of 36.57 parts of Ultem 1000 and 1.93 parts ofSiltem D9000.

The results of air separation testing of the membranes in Examples 1through 4 are summarized in Table 3. These data show that the propertiesof hollow fibers can be controlled by varying the ratio of Ultem toSiltem in the spin dope, making this a useful method of preparingmembranes for fluid or gas separation applications.

COMPARATIVE EXAMPLE 5

A hollow fiber membrane was prepared and tested as described in Example1 except that the polymer component of the spinning dope consisted of38.5 parts of Ultem 1000 only. The gas separation performance of thismembrane that is not part of the present invention is listed in Table 3.

As can be seen from the results summarized in Table 3, the hollow fibermembranes prepared according to the methods of the present inventionexhibit improved gas permeation characteristics.

                  TABLE 3    ______________________________________    Air separation properties of coated    Ultem/Siltem blend hollow fiber membranes              Siltem/Ultem  O.sub.2 P/t*    Example No.              ratio         × 10.sup.-5                                    α O.sub.2 /N.sub.2    ______________________________________    1         0.006         3.75    2.24    2         0.013         5.39    2.61    3         0.025         9.48    2.41    4         0.05          10.03   2.38    5         0             1.77    3.06    ______________________________________     *in units of cm.sup.3 (STP)/cm.sup.2 cmHg sec

The terms and descriptions used herein are preferred embodiments setforth by way of illustration only, and are not intended as limitationson the many variations which those skilled in the art will recognize tobe possible in practicing the present invention as defined by theclaims.

What is claimed is:
 1. A process for making an anisotropic gasseparation membrane, said process comprising:(a) forming a blend of atleast two polymers in at least one solvent, wherein at least one of saidpolymers being a low surface energy polymer having a measured surfaceenergy that is at least 10% lower than the measured surface energy ofthe other polymers in the blend; (b) extruding said blend, as a hollowfiber, through a gaseous atmosphere and into a liquid medium whereinsaid fiber solidifies as an anisotropic gas separation membrane; and (c)recovering the solidified fiber anisotropic gas separation membrane,wherein said low surface energy polymer comprises less than 20% byweight of the overall membrane material and is concentrated in a surfacediscriminating layer.
 2. The process according to claim 1, wherein saidlow surface energy polymer comprises polyimide, polyester orpolycarbonate.
 3. The process according to claim 1, wherein said lowsurface energy polymer contains siloxane or fluorocarbon groups.
 4. Theprocess according to claim 1, wherein the bulk of said membrane materialcomprises a polyetherimide or polysulfone.
 5. The process according toclaim 2, wherein said gaseous atmosphere is maintained at asubatmospheric pressure.
 6. The process according to claim 1, whereinsaid membrane is a hollow fiber membrane.
 7. The process of claim 1,wherein said low surface energy polymer comprises less than 10% byweight of the overall membrane material.
 8. The process of claim 1,wherein said low surface energy polymer comprises less than 5% by weightof the overall membrane material.